Studies are proposed on the catalytic cooperativity among enzymes and enzyme subunits involved in oxidative phosphorylation by mitochondrial and chloroplast membranes, by ion transport ATPases, and by purified multisubunit enzymes. An important probe will be the use of oxygen interchange from HOH into torsionally symmetrical groups, such as phosphate and carboxylate, which may reveal rates of reaction of bound substrates. Also included will be a combination of rapid mixing-quenching techniques and measurements of patterns of labeling and release of membrane-bound nucleotides. Experiments with pyrophosphatase will emphasize development of methodology for following disappearance of 18O from fully labeled phosphate, together with computer assisted evaluations of statistical patterns expected for various enzyme mechanisms. Other studies will probe possibilities of overlooked catalytic cooperativity in glyceraldehyde 3-phosphate dehydrogenase. With the purified mitochondrial F1-ATPase, measurements were made of the possible effect of ATP in promoting release of the tightly but slowly bound and released Pi. Such studies will have as an aim gaining additional information as to whether ATP binding at one site may promote substrate release at another.